The rotating shafts of gas turbine engines are supported from a nonrotating support structure by two or more arrays of ball bearings or roller bearings. In many engines, one of the bearing arrays is enclosed within a bearing compartment located radially inward of the engine combustion chamber, the compartment being bounded in part by an outer wall circumscribing the engine shaft. A quantity of compressed air from the engine compressor is ducted past the bearing compartment and into the engine turbines where the air is used to cool the turbines. Although this air is cool relative to the turbines, it is at an elevated temperature compared to the bearing compartment. The presence of this hot air and the proximity of the bearing compartment to the combustor create a harsh, high temperature operating environment for the bearing compartment. A sealing arrangement including a self adjusting face seal and a piston ring seal are used to separate the interior of the bearing compartment from the surrounding local environment.
A lubricant such as oil is supplied to the compartment to lubricate and cool the bearing array and to lubricate the face seal. The face seal minimizes the leakage of oil out of the compartment, and the piston ring seal minimizes the leakage of hot air into the compartment. Minimization of hot air ingress is important because the hot air can ignite and support combustion of oil in the compartment, resulting in an engine fire. Oil leakage out of the compartment can have similar consequences, especially if a quantity of oil accumulates outside the compartment. Moreover, the presence of hot air inside of or immediately adjacent to the compartment encourages the formation of coke within the compartment. Coke is a residue which forms when oil or other hydrocarbons are exposed to high temperatures. Coke contaminates the self adjusting mechanism of the face seal thereby interfering with its smooth operation and compromising the effectiveness of the face seal. In extreme cases, accumulations of coke can cause the mechanism to bind, damaging the mechanism or adjacent parts.
In many engines which have been designed since the late 1960's, the above described sealing arrangement is augmented by a buffering system. In a buffered system, cool buffer air is introduced into a buffer air passage adjacent to the bearing compartment wall. The buffer air flows through the passage and is continuously replenished. The buffer air discharges from the inner end of the passage into an annular buffer cavity where it serves as an additional barrier against the leakage of the turbine cooling air into the bearing compartment. More complete descriptions of buffering systems are found in U.S. Pat. No. 4,542,623 issued to Hovan et al., U. S. Pat. No. 4,561,246 issued to Hovan and U.S. Pat. No. 4,709,545 issued to Stevens et al.
Engines designed prior to the late 1960's (or derived from such earlier designs) run at somewhat lower temperatures than the more modern engines and therefore buffering is not used to protect the bearing compartment. Instead, an insulation blanket is positioned radially outward of the bearing compartment and held in place by a heatshield. It has been determined that the construction of the heatshield and the bearing compartment wall and the presence of the insulating blanket leads to an accumulation of oil in the blanket and exposes the accumulated oil to a continuously flowing stream of hot air. This presents a fire risk since the hot air can ignite and support combustion of the oil. The fire risk develops slowly over time, and therefore the risk is mitigated by carrying out periodic inspections of the hardware and monitoring the temperature of the lubricating oil. Unfortunately, such monitoring and inspection is time consuming and costly, particularly for a bearing compartment which is radially inward of the combustor and therefore not readily accessible without significant disassembly of the engine.
Clearly, it is desirable to eliminate the risk of fire, and to do so in a way that does not involve costly and time consuming inspections. Due to the limited accessibility of the bearing compartment, it is also desirable to minimize the likelihood of damage to components located within or near the bearing compartment.
What is needed is a bearing compartment cover that eliminates the risk of fire and enhances the durability of the bearing compartment components.